There is considered application of a nitride semiconductor to a semiconductor device with high withstand voltage and high output power, utilizing characteristics such as high saturation electron speed and wide band gap. For example, the band gap of GaN as the nitride semiconductor is 3.4 eV, which is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV), and thus GaN has high breakdown electric field intensity. Accordingly, GaN is quite promising as a material of a semiconductor device for power supply that obtains high voltage operation and high output power.
As a semiconductor device using the nitride semiconductor, there have been made numerous reports on a field effect transistor, particularly a high electron mobility transistor (HEMT). For example, among GaN-based HEMTs (GaN-HEMTs), an AlGaN/GaN.HEMT using GaN as an electron transit layer and using AlGaN as an electron supply layer is attracting attention. In the AlGaN/GaN.HEMT, a strain resulted from a lattice constant difference between GaN and AlGaN occurs in AlGaN. Two-dimensional electron gas (2DEG) of high concentration is obtained from piezoelectric polarization caused by the strain and spontaneous polarization of AlGaN. Accordingly, the AlGaN/GaN.HEMT is expected as a high efficiency switch element and a high-withstand-voltage electric power device for electric vehicle, or the like.
Patent Document 1: Japanese Laid-open Patent Publication No. 2002-16087
Patent Document 2: Japanese Laid-open Patent Publication No. 2013-77638
Patent Document 3: Japanese Laid-open Patent Publication No. 2003-229439